when the store of force in our brain has been already considerably drawn upon by the whole day's work. If no recovery by sleep is enjoyed, or it is imperfect, the consequences will invariably make themselves evident the next day in a depression of mental vigor as well as in a rise in the personal susceptibility to fatigue. The rapidity with which one of the persons I experimented upon could perform his tasks in addition sank about a third after a night journey by railway with insufficient sleep. Another experimenter could detect the effects of keeping himself awake all night in a gradual decrease of vigor lasting through four days. This observation was all the more surprising, because the subject was not conscious of the long duration of the disturbance, and was first made aware of it incidentally by the results of continued measurements on the causes of the manifestations of fatigue.
These experiments admonish us to give special attention to the question of sleep with men who work with their minds. This is of more especial importance for the growing generation, because the susceptibility to fatigue, and consequently the need of sleep, are much greater in the youthful brain than in that of adults. The average duration of sleep has been studied by Axel Key in Swedish pupils of different ages. He found that it ranges from nine hours in children ten years old down to seven hours in pupils of eighteen years. Children ten years old were found who slept only six, and some of seventeen or eighteen years who had to satisfy themselves with four hours!—a result which is really astonishing. Axel Key is certainly right when he assumes that the mass of Swedish school children of all ages are daily deprived of one or two hours of their needed sleep, to say nothing of those unfortunate ones who can sleep only half the time or less which is required for their healthy mental and bodily development.
The amounts of sleep required by different men are very various, for they are dependent on the deepness of the slumber. There are persons who sleep so soundly that a surprisingly short time spent in sleeping is enough for them. On the other hand, we know that for many idiosyncrasies a length of sleep which is quite enough for the average of men is much too short.
Besides sleep, which limits the waste for a certain time and favors the restoration of what has been consumed, we need for the maintenance of our working strength the assimilation of food. By means of food the substances are introduced to the tissues which they require for their constant renewal. Sleep alone can indeed retard for a long time the continued destruction of the organs by the processes of life, as it does in the hibernation of animals; but there comes a point at last when only the introduction of fresh restorative matter can assure the continued main-
